SYNOPSIS

DESCRIPTION

These functions implement voluntary context switching. tsleep() and
sleep() are used throughout the kernel whenever processing in the current
context can not continue for any of the following reasons:
• The current process needs to await the results of a pending I/O
operation.
• The current process needs resources (e.g., memory) which are
temporarily unavailable.
• The current process wants access to data structures which are
locked by other processes.
The function wakeup() is used to notify sleeping processes of possible
changes to the condition that caused them to go to sleep. Typically, an
awakened process will -- after it has acquired a context again -- retry
the action that blocked its operation to see if the "blocking" condition
has cleared.
The bpendsleep label can be used as a break-point to debug a process com-
ing back from tsleep().
The tsleep() function takes the following arguments:
ident An identifier of the "wait channel" representing the resource
for which the current process needs to wait. This typically is
the virtual address of some kernel data structure related to
the resource for which the process is contending. The same
identifier must be used in a call to wakeup() to get the pro-
cess going again. ident should not be NULL.
priority The process priority to be used when the process is awakened
and put on the queue of runnable processes. This mechanism is
used to optimize "throughput" of processes executing in kernel
mode. If the flag PCATCH is OR'ed into priority the process
checks for posted signals before and after sleeping.
wmesg A pointer to a character string indicating the reason a process
is sleeping. The kernel does not use the string, but makes it
available (through the process structure field p_wmesg) for
user level utilities such as ps(1).
timo If non-zero, the process will sleep for at most timo/hz
seconds. If this amount of time elapses and no wakeup(ident)
has occurred, and no signal (if PCATCH was set) was posted,
tsleep() will return EWOULDBLOCK.
The sleep() function puts the process in an uninterruptible sleep. It is
functionally equivalent to:
tsleep(ident, priority & PRIMASK, 0, 0)
The wakeup() function will mark all processes which are currently sleep-
ing on the identifier ident as runnable. Eventually, each of the
processes will resume execution in the kernel context, causing a return
from [t]sleep(). Note that processes returning from sleep should always
re-evaluate the conditions that blocked them, since a call to wakeup()
merely signals a possible change to the blocking conditions. For example,
when two or more processes are waiting for an exclusive lock, only one of
them will succeed in acquiring the lock when it is released. All others
will have to go back to sleep and wait for the next opportunity.

RETURN VALUES

tsleep() returns 0 if it returns as a result of a wakeup(). If a tsleep()
returns as a result of a signal, the return value is ERESTART if the sig-
nal has the SA_RESTART property (see sigaction(2)), and EINTR otherwise.
If tsleep() returns as a result of a timeout, the return value is
EWOULDBLOCK.